In the field of data communications, the exploding demand for broadband network communications has spread to a number of different forms of communications media, e.g., cable, satellite, twisted copper pair, optical fiber, etc. Currently, half the households in the United States have high-speed broadband Internet access. Recently, the cable modem has become the broadband connection of choice for many Internet users, being preferred over the nearest rival broadband technology, Digital Subscriber Line (DSL), by a significant margin.
Cable modems are well known in the art. A cable modem is a type of modem that provides access to a data signal sent over the cable television (CATV) infrastructure. Cable modems are primarily used to deliver broadband Internet access, taking advantage of unused bandwidth on a cable television network. In 2005 there were over 22.5 million cable modem users in the United States alone.
A cable modem is a network appliance that enables high speed data connections to the internet via data services provided by the local cable company. Data from the home is sent upstream on a carrier that operates on the 5 MHz to 42 MHz band of the cable spectrum. Downstream data is carried on an 88 MHz to 860 MHz band. The cable modem system can have additional networking features such as Voice over IP (VoIP), wireless connectivity or network switch or hub functionality.
The term cable Internet access refers to the delivery of Internet service over the cable television infrastructure. The proliferation of cable modems, along with DSL technology, has enabled broadband Internet access in many countries. The bandwidth of cable modem service typically ranges from 3 Mbps up to 40 Mbps or more. The upstream bandwidth on residential cable modem service usually ranges from 384 kbps to 30 Mbps or more. In comparison, DSL tends to offer less speed and more variance between service packages and prices. Service quality is also far more dependent on the client's location in relation to the telephone company's nearest central office or Remote Terminal.
Users in a neighborhood share the available bandwidth provided by a single coaxial cable line. Therefore, connection speed varies depending on how many people are using the service at the same time. In most areas this has been eliminated due to redundancy and fiber networks.
With the advent of Voice over IP telephony, cable modems are also be used to provide telephone service. Many people who have cable modems have opted to eliminate their Plain Old Telephone Service (POTS). An alternative to cable modems is the Embedded Multimedia Terminal Adapter (EMTA). An EMTA allows multiple service operators (MSOs) to offer both High Speed Internet and VoIP through a single piece of customer premise equipment. A multiple system operator is an operator of multiple cable television systems.
Many cable companies have launched Voice over Internet Protocol (VoIP) phone service, or digital phone service, providing consumers a true alternative to standard telephone service. Digital phone service takes the analog audio signals and converts them to digital data that can be transmitted over the fiber optic network of the cable company. Cable digital phone service is currently available to the majority of U.S. homes with a large number of these homes subscribing to the service. The number of homes subscribing is currently growing by hundreds of thousands each quarter. One significant benefit of digital phone service is the substantial consumer savings, with one recent study saying residential cable telephone consumers could save an average of $135 or more each year.
The Data Over Cable Service Interface Specification (DOCSIS) compliant cable modems have been fueling the transition of cable television operators from a traditional core business of entertainment programming to a position as full-service providers of video, voice, and data telecommunications services.
The latest DOCSIS specification, DOCSIS 3.0, include a number of enhancements, most notably, channel bonding and support for IPv6. Channel bonding provides cable operators with a flexible way to increase upstream and downstream throughput to customers, with data rates in the hundreds of megabits and potentially gigabits per second. DOCSIS 3.0 increases the capacity of cable modems to a minimum of 160 Mbps downstream to customers and to a minimum of 120 Mbps upstream.
Cable systems transmit digital data signals over radio frequency (RF) carrier signals. To provide two-way communication, one carrier signal carries data in the downstream direction from the cable network to the customer and another carrier signal carries data in the upstream direction from the customer to the cable network. Cable modems are devices located at the subscriber premises that functions to convert digital information into a modulated RF signal in the upstream direction, and to convert the RF signals to digital information in the downstream direction. A cable modem termination system (CMTS) performs the opposite operation for multiple subscribers at the cable operator's head-end.
Typically, several hundreds of users share a 6 MHz downstream channel and one or more upstream channels. The downstream channel occupies the space of a single television transmission channel in the cable operator's channel lineup. It is compatible with digital set top MPEG transport stream modulation (64 or 256 QAM), and provides up to 40 Mbps. A media access control (MAC) layer coordinates shared access to the upstream bandwidth.
In order to provide faster data rates to customers, DOCSIS 3.0 introduces the concept of bonding several physical downstream channels into one virtual high speed pipe. Channel bonding is a load-sharing technique for logically combining multiple DOCSIS channels. DOCSIS 3.0 defines channel bonding for both the upstream and downstream directions. For downstream channel bonding, each downstream DOCSIS channel carries a payload of approximately 38 Mbps (50 Mbps with EuroDOCSIS). Load sharing traffic across multiple channels allows a maximum throughput of up to n×38 Mbps (or n×50 Mbps), with n representing the number of channels being bonded. A separate 6 MHz or 8 MHz frequency is used for each of the bonded channels. Upstream channel bonding is possible to implement, for example, over four channels, 10 to 30 Mbps each, for a total of 40 to 120 Mbps of shared throughput.
In DOCSIS 3.0 each active upstream channel has a different dynamic range window in which the channel can transmit. A diagram illustrating the dynamic range window for each upstream transmit channel in DOCSIS 3.0 system is shown in FIG. 1. The top of each dynamic window is determined according to the burst profile of the particular channel (i.e. TDMA/SCDMA, constellation which is different for each channel) and to a parameter known as Pload—min—set. The Pload—min—set parameter 66 is normally sent as a command from the CMTS and is the same for all channels. The DOCSIS 3.0 specification ensures a global quiet time (i.e. time when all channels are quiet) only after a new Pload—min—set parameter is received from the CMTS.
The upper limit of each channel is Phi 65 (i.e. Phi—1, Phi—2, Phi—3, Phi—4, for channels 1-4, respectively). The Pload—min—set parameter 66 determines the upper point of the dynamic range window 67 which is 12 dB. The lower limit of each dynamic range window is thus 12 dB down from the upper limit and is referred to as Plow—multi 68 (i.e. Plow multi 1, Plow multi 2, Plow multi 3, Plow—multi—4, for channels 1-4, respectively). For example, assuming a Phi of 60 dBmV and a Pload—min—set of 5 dBmV, results in an upper limit of the dynamic range window of 55 dBmV and a lower limit of 43 dBmV. The working point for each channel is within the dynamic range window.
To reduce the cost of a cable modem system, it is common to include only a single programmable gain amplifier (PGA) for all four upstream channels, as the cost of the amplifier hardware component is relatively substantial. Typically, the upstream system includes several channels whose digital outputs are summed before entering a DAC. Each channel is duplex and operates at a different frequency. The analog output of the DAC is amplified by the single amplifier (e.g., PGA) before being coupled to the CATV cable. Each channel comprises a digital attenuator for adjusting the output signal level before entering the DAC.
With an upstream system implementation that includes only a single PGA for all channels, the gain of the PGA can only be changed when the CMTS grants a global reconfiguration time (i.e. global quiet time). Thus, each setting of the PGA must be suitable for all possible gain configurations the cable modem may be commanded to set, until the next global reconfiguration time. This implies that one or more channels may have a large backoff part of the time (or all of the time) when their output signals enter the DAC. This results in the PGA gain being set at a sub-optimal point the majority of the time. Note that the CMTS re-configuration time events as indicated in the DOCSIS specification are mandatory after a change in Pload—min—set, regardless of the frequency of their occurrence.
Having a large backoff in the channels results in significant interference and noise as a result of the attenuation at the input to the DAC. The larger the attenuation, the more noise that is generated. Ideally, the output signals from the channels should enter the DAC with minimum backoff to achieve the highest input signal level. Note that some backoff is required in order to prevent clipping of the output signal.
In prior art DOCSIS systems, with only a single channel, all gain changes could be made (1) at the PGA and (2) by changing the digital attenuation at the input to the DAC (i.e. changing the digital gain). The CMTS, does, however, issue gain commands on a per channel basis. In response, the modem sets the digital attenuator in front of the DAC but only when the specific channel is quiet. The PGA must accept the highest gain for each channel that is permissible, i.e. the highest gain in the dynamic range window, while also taking into account a possible change to Phi. The value of Pload—min—set sent in a command from the CMTS determines the placement of the dynamic range window. Each channel has the same size window, but at a different location. The DOCSIS specification dictates that a new Pload—min—set is set at a global quiet time. It is at this time, the PGA gain can be changed.
In response to requests from the CMTS for gain changes for a single channel (i.e. within the dynamic range window), the cable modem changes the digital gain in front of the DAC. This results in sub-optimal operation when all upstream channels are combined. Although the CMTS is able to change the Pload—min—set, it cannot do it often. Ideally, the cable modem always transmits at a maximum signal level. It is thus desirable to change the PGA gain every time the gain of a channel is to be changed.
It is preferable for the channel output signals to enter the DAC with the smallest backoff possible in order to minimize frequency spurs which are limited by the DOCSIS standard. To minimize spurs, the full scale is set as close as possible to the actual transmit power. The ability to change the PGA gain setting more frequently would thus enable gain changes, including both changes at the PGA and digital gain changes) to be more optimized with respect to back off, resulting in better performance.